Method of manufacturing a nuclear fuel assembly

ABSTRACT

Nuclear fuel assemblies include fuel elements that are sintered or cast into billets and co-extruded into a spiral, multi-lobed shape. The fuel kernel may be a metal alloy of metal fuel material and a metal-non-fuel material, or ceramic fuel in a metal non-fuel matrix. The fuel elements may use more highly enriched fissile material while maintaining safe operating temperatures. Such fuel elements according to one or more embodiments may provide more power at a safer, lower temperature than possible with conventional uranium oxide fuel rods. The fuel assembly may also include a plurality of conventional UO2 fuel rods, which may help the fuel assembly to conform to the space requirements of conventional nuclear reactors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/900,071, filed Feb. 20, 2018, which is a U.S. National Stageapplication under 35 USC § 371 of PCT/US2011/036034 filed 11 May 2011,which claims priority benefit under the Paris Convention from U.S.Provisional Application No. 61/333,467, filed May 11, 2010, U.S.Provisional Application No. 61/393,499, filed Oct. 15, 2010, and U.S.Provisional Application No. 61/444,990, filed Feb. 21, 2011, all threeof which are titled “METAL FUEL ASSEMBLY,” the entire contents of whichare hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to nuclear fuel assemblies usedin the core of a nuclear reactor, and relates more specifically to metalnuclear fuel elements.

Description of Related Art

U.S. Patent Application Publication No. 2009/0252278 A1, the entirecontents of which are incorporated herein by reference, discloses anuclear fuel assembly that includes seed and blanket sub-assemblies. Theblanket sub-assembly includes thorium-based fuel elements. The seedsub-assembly includes Uranium and/or Plutonium metal fuel elements usedto release neutrons, which are captured by the Thorium blanket elements,thereby creating fissionable U-233 that burns in situ and releases heatfor the nuclear power plant.

Conventional nuclear power plants typically use fuel assemblies thatinclude a plurality of fuel rods that each comprise uranium oxide fuelin a cylindrical tube.

SUMMARY OF EMBODIMENTS OF THE INVENTION

The surface area of the cylindrical tube of conventional fuel rodslimits the amount of heat that can be transferred from the rod to theprimary coolant. To avoid overheating the fuel rod in view of thelimited surface area for heat flux removal, the amount of fissilematerial in these uranium oxide fuel rods or mixed oxide (plutonium anduranium oxide) fuel rods has conventionally been substantially limited.

One or more embodiments of the present invention overcome variousdisadvantages of conventional uranium oxide fuel rods by replacing themwith all metal, multi-lobed, powder metallurgy co-extruded fuel rods(fuel elements). The metal fuel elements have significantly more surfacearea than their uranium oxide rod counterparts, and therefore facilitatesignificantly more heat transfer from the fuel element to the primarycoolant at a lower temperature. The spiral ribs of the multi-lobed fuelelements provide structural support to the fuel element, which mayfacilitate the reduction in the quantity or elimination of spacer gridsthat might otherwise have been required. Reduction in the quantity orelimination of such spacer grids advantageously reduces the hydraulicdrag on the coolant, which can improve heat transfer to the coolant.Because the metal fuel elements may be relatively more compact thantheir conventional uranium oxide fuel rod counterparts, more spacewithin the fuel assembly is provided for coolant, which again reduceshydraulic drag and improves heat transfer to the coolant. The higherheat transfer from the metal fuel rods to the coolant means that it ispossible to generate more heat (i.e., power), while simultaneouslymaintaining the fuel elements at a lower operating temperature due tothe considerably higher thermal conductivity of metals versus oxides.Although conventional uranium oxide or mixed oxide fuel rods typicallyare limited to fissile material loading of around 4-5% due tooverheating concerns, the higher heat transfer properties of the metalfuel elements according to various embodiments of the present inventionenable significantly greater fissile material loadings to be used whilestill maintaining safe fuel performance. Ultimately, the use of metalfuel elements according to one or more embodiments of the presentinvention can provide more power from the same reactor core thanpossible with conventional uranium oxide or mixed oxide fuel rods.

The use of all-metal fuel elements according to one or more embodimentsof the present invention may advantageously reduce the risk of fuelfailure because the metal fuel elements reduce the risk of fission gasrelease to the primary coolant, as is possible in conventional uraniumoxide or mixed oxide fuel rods.

The use of all-metal fuel elements according to one or more embodimentsof the present invention may also be safer than conventional uraniumoxide fuel rods because the all-metal design increases heat transferwithin the fuel element, thereby reducing temperature variations withinthe fuel element, and reducing the risk of localized overheating of thefuel element.

One or more embodiments of the present invention provide a method ofmanufacturing a fuel assembly for use in a core of a nuclear powerreactor. The method includes manufacturing each of a plurality ofelongated fuel elements by mixing fuel material with metal non-fuelmaterial, wherein the fuel material comprises fissile material. Themixed fuel material and metal non-fuel material is cast to create a fuelcore stock. The fuel core stock is surrounded with a cladding material.The fuel core stock and cladding material are co-extruded to create thefuel element. The method further includes mounting the plurality ofelongated fuel elements to a frame of the fuel assembly.

These and other aspects of various embodiments of the present invention,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. In one embodiment of the invention, the structuralcomponents illustrated herein are drawn to scale. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention. In addition, it should be appreciatedthat structural features shown or described in any one embodiment hereincan be used in other embodiments as well. As used in the specificationand in the claims, the singular form of “a”, “an”, and “the” includeplural referents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the present invention aswell as other objects and further features thereof, reference is made tothe following description which is to be used in conjunction with theaccompanying drawings, where:

FIG. 1 is a cross-sectional view of a fuel assembly according to anembodiment of the present invention, the cross-section being taken in aself-spacing plane;

FIG. 2 is a cross-sectional view of the fuel assembly of FIG. 1 , thecross-section being taken in a plane that is shifted by ⅛ of a twist ofthe fuel elements from the view in FIG. 1 ;

FIG. 3 is a cross-sectional view of the fuel assembly of FIG. 1 , takenin a plane that is parallel to the axial direction of the fuel assembly;

FIG. 4 is a perspective view of a fuel element of the fuel assembly ofFIG. 1 ;

FIG. 5 is a cross-sectional view of the fuel element in FIG. 3 ;

FIG. 6 is a cross-sectional view of the fuel element in FIG. 3 ,circumscribed within a regular polygon;

FIG. 7A is an end view of a fuel assembly according to an alternativeembodiment, for use in a pressurized heavy water reactor;

FIG. 7B is a partial side view of the fuel assembly of FIG. 7A;

FIG. 8 is a diagram of a pressurized heavy water reactor using the fuelassembly illustrated in FIGS. 7A and 7B

FIG. 9 is a cross-sectional view of the fuel element in FIG. 3 ; and

FIG. 10 is a cross-sectional view of a fuel assembly according to anembodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

FIGS. 1-3 illustrate a fuel assembly 10 according to an embodiment ofthe present invention. As shown in FIG. 3 , the fuel assembly 10comprises a plurality of fuel elements 20 supported by a frame 25.

As shown in FIG. 3 , the frame 25 comprises a shroud 30, guide tubes 40,an upper nozzle 50, a lower nozzle 60, a lower tie plate 70, an uppertie plate 80, and/or other structure(s) that enable the assembly 10 tooperate as a fuel assembly in a nuclear reactor. One or more of thesecomponents of the frame 25 may be omitted according to variousembodiments without deviating from the scope of the present invention.

As shown in FIG. 3 , the shroud 25 mounts to the upper nozzle 50 andlower nozzle 60. The lower nozzle 60 (or other suitable structure of theassembly 10) is constructed and shaped to provide a fluid communicationinterface between the assembly 10 and the reactor 90 into which theassembly 10 is placed so as to facilitate coolant flow into the reactorcore through the assembly 10 via the lower nozzle 60. The upper nozzle50 facilitates direction of the heated coolant from the assembly 10 tothe power plant's steam generators (for PWRs), turbines (for BWRs), etc.The nozzles 50, 60 have a shape that is specifically designed toproperly mate with the reactor core internal structure.

As shown in FIG. 3 , the lower tie plate 70 and upper tie plate 80 arepreferably rigidly mounted (e.g., via welding, suitable fasteners (e.g.,bolts, screws), etc.) to the shroud 30 or lower nozzle 60 (and/or othersuitable structural components of the assembly 10).

Lower axial ends of the elements 20 form pins 20 a that fit into holes70 a in the lower tie plate 70 to support the elements 20 and helpmaintain proper element 20 spacing. The pins 20 a mount to the holes 70a in a manner that prevents the elements 20 from rotating about theiraxes or axially moving relative to the lower tie plate 70. Thisrestriction on rotation helps to ensure that contact points betweenadjacent elements 20 all occur at the same axial positions along theelements 20 (e.g., at self-spacing planes discussed below). Theconnection between the pins 20 a and holes 70 a may be created viawelding, interference fit, mating non-cylindrical features that preventrotation (e.g., keyway and spline), and/or any other suitable mechanismfor restricting axial and/or rotational movement of the elements 20relative to the lower tie plate 70. The lower tie plate 70 includesaxially extending channels (e.g., a grid of openings) through whichcoolant flows toward the elements 20.

Upper axial ends of the elements 20 form pins 20 a that freely fit intoholes 80 a in the upper tie plate 80 to permit the upper pins 20 a tofreely axially move upwardly through to the upper tie plate 80 whilehelping to maintain the spacing between elements 20. As a result, whenthe elements 20 axially grow during fission, the elongating elements 20can freely extend further into the upper tie plate 80.

As shown in FIG. 4 , the pins 70 a transition into a central portion ofthe element 20.

FIGS. 4 and 5 illustrate an individual fuel element/rod 20 of theassembly 10. As shown in FIG. 5 , the elongated central portion of thefuel element 20 has a four-lobed cross-section. A cross-section of theelement 20 remains substantially uniform over the length of the centralportion of the element 20. Each fuel element 20 has a fuel kernel 100,which includes a refractory metal and fuel material that includesfissile material.

A displacer 110 that comprises a refractory metal is placed along thelongitudinal axis in the center of the fuel kernel 100. The displacer110 helps to limit the temperature in the center of the thickest part ofthe fuel element 20 by displacing fissile material that would otherwiseoccupy such space and minimize variations in heat flux along the surfaceof the fuel element. According to various embodiments, the displacer 110may be eliminated altogether.

As shown in FIG. 5 , the fuel kernel 100 is enclosed by a refractorymetal cladding 120. The cladding 120 is preferably thick enough, strongenough, and flexible enough to endure the radiation-induced swelling ofthe kernel 100 without failure (e.g., without exposing the kernel 100 tothe environment outside the cladding 120). According to one or moreembodiments, the entire cladding 120 is at least 0.3 mm, 0.4 mm, 0.5 mm,and/or 0.7 mm thick. According to one or more embodiments, the cladding120 thickness is at least 0.4 mm in order to reduce a chance ofswelling-based failure, oxidation based failure, and/or any otherfailure mechanism of the cladding 120.

The cladding 120 may have a substantially uniform thickness in theannular direction (i.e., around the perimeter of the cladding 120 asshown in the cross-sectional view of FIG. 5 ) and over theaxial/longitudinal length of the kernel 100 (as shown in FIG. 4 ).Alternatively, as shown in FIG. 5 , according to one or moreembodiments, the cladding 120 is thicker at the tips of the lobes 20 bthan at the concave intersection/area 20 c between the lobes 20 b. Forexample, according to one or more embodiments, the cladding 120 at thetips of the lobes 20 b is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%,80%, 90%, 100%, 125%, and/or 150% thicker than the cladding 120 at theconcave intersections/areas 20 c. The thicker cladding 120 at the tipsof the lobes 20 b provides improved wear resistance at the tips of thelobes 20 b where adjacent fuel elements 20 touch each other at theself-spacing planes (discussed below).

The refractory metal used in the displacer 110, the fuel kernel 100, andthe cladding 120 comprises zirconium according to one or moreembodiments of the invention. As used herein, the term zirconium meanspure zirconium or zirconium in combination with other alloy material(s).However, other refractory metals may be used instead of zirconiumwithout deviating from the scope of the present invention (e.g.,niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium,chromium, zirconium, hafnium, ruthenium, osmium, iridium, and/or othermetals). As used herein, the term “refractory metal” means anymetal/alloy that has a melting point above 1800 degrees Celsius (2073K).

Moreover, in certain embodiments, the refractory metal may be replacedwith another non-fuel metal, e.g., aluminum. However, the use of anon-refractory non-fuel metal is best suited for reactor cores thatoperate at lower temperatures (e.g., small cores that have a height ofabout 1 meter and an electric power rating of 100 MWe or less).Refractory metals are preferred for use in cores with higher operatingtemperatures.

As shown in FIG. 5 , the central portion of the fuel kernel 100 andcladding 120 has a four-lobed profile forming spiral spacer ribs 130.The displacer 110 may also be shaped so as to protrude outwardly at theribs 130 (e.g., corners of the square displacer 110 are aligned with theribs 130). According to alternative embodiments of the presentinvention, the fuel elements 20 may have greater or fewer numbers ofribs 130 without deviating from the scope of the present invention. Forexample, as generally illustrated in FIG. 5 of U.S. Patent ApplicationPublication No. 2009/0252278 A1, a fuel element may have threeribs/lobes, which are preferably equally circumferentially spaced fromeach other. The number of lobes/ribs 130 may depend, at least in part,on the shape of the fuel assembly 10. For example, a four-lobed element20 may work well with a square cross-sectioned fuel assembly 10 (e.g.,as is used in the AP-1000). In contrast, a three-lobed fuel element maywork well with a hexagonal fuel assembly (e.g., as is used in the VVER).

FIG. 9 illustrates various dimensions of the fuel element 20 accordingto one or more embodiments. According to one or more embodiments, any ofthese dimensions, parameters and/or ranges, as identified in the belowtable, can be increased or decreased by up to 5%, 10%, 15%, 20%, 25%,30%, 40%, 50%, or more without deviating from the scope of the presentinvention.

Fuel Element 20 Parameter Symbol Example Values Unit Circumscribeddiameter D 9-14 (e.g., 12.3, 12.4, 12.5, 12.6) mm Lobe thickness Δ2.5-3.8 (e.g., 2.5, 2.6, 2.7, 2.8, mm 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5,3.6, 3.7, 3.8), variable Minimum cladding thickness δ 0.4-1.2 (e.g.,0.4, 0.5, 0.6, 0.7, mm 0.8, 0.9, 1.0, 1.1, 1.2) Cladding thickness atthe lobe δ^(max) 0.4-2.2 (e.g., 0.4, 0.5, 0.6, 0.7, mm 0.9, 1.0, 1.1,1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2), 1.5δ, 2δ, 2.5δAverage cladding thickness 0.4-1.8 (e.g., 0.4, 0.5, 0.6, 0.7, mm 0.8,0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8), at least 0.4, 0.5, or0.6 Curvature radius of cladding at lobe r Δ/2, Δ/1.9, variable mmperiphery Curvature radius of fuel kernel at lobe r_(f) 0.5-2.0 (e.g.,0.5, 0.6, 0.7, 0.8, mm periphery 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6,1.7, 1.8, 1.9, 2.0), (Δ-2δ)/2, variable Radius of curvature betweenadjacent R 2-5 (e.g., 2, 3, 4, 5), variable mm lobes Central displacerside length a 1.5-3.5 (e.g., 1.5, 1.6, 1.7, mm 1.8, 1.9, 2.0, 2.1, 2.2,2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5) Fuelelement perimeter 25-60 (e.g., 25, 30, 35, 40, 45, 50, 55, 60) mm Fuelelement area 50-100 (e.g., 50, 60, 70, 80, 90, 100) mm² Fuel kernelarea, mm² 30-70 (e.g., 30, 40, 50, 60, 70) mm² Enrichment <19.7 w/o Ufraction <25 v/o

As shown in FIG. 4 , the displacer 110 has a cross-sectional shape of asquare regular quadrilateral with the corners of the square regularquadrilateral being aligned with the ribs 130. The displacer 110 forms aspiral that follows the spiral of the ribs 130 so that the corners ofthe displacer 110 remain aligned with the ribs 130 along the axiallength of the fuel kernel 100. In alternative embodiments with greateror fewer ribs 130, the displacer 110 preferably has the cross-sectionalshape of a regular polygon having as many sides as the element 20 hasribs.

As shown in FIG. 6 , the cross-sectional area of the central portion ofthe element 20 is preferably substantially smaller than the area of asquare 200 in which the tip of each of the ribs 130 is tangent to oneside of the square 200. In more generic terms, the cross-sectional areaof an element 20 having n ribs is preferably smaller than the area of aregular polygon having n sides in which the tip of each of the ribs 130is tangent to one side of the polygon. According to various embodiments,a ratio of the area of the element 20 to the area of the square (orrelevant regular polygon for elements 20 having greater or fewer thanfour ribs 130) is less than 0.7, 0.6, 0.5, 0.4, 0.35, 0.3. As shown inFIG. 1 , this area ratio approximates how much of the available spacewithin the shroud 30 is taken up by the fuel elements 20, such that alower ratio means that more space is advantageously available forcoolant, which also acts as a neutron moderator and which increases themoderator-to-fuel ratio (important for neutronics), reduces hydraulicdrag, and increases the heat transfer from the elements 20 to thecoolant. According to various embodiments, the resulting moderator tofuel ratio is at least 2.0, 2.25, 2.5, 2.75, and/or 3.0 (as opposed to1.96 when conventional cylindrical uranium oxide rods are used).Similarly, according to various embodiments, the fuel assembly 10 flowarea is increased by over 16% as compared to the use of one or moreconventional fuel assemblies that use cylindrical uranium oxide rods.The increased flow area may decrease the coolant pressure drop throughthe assembly 10 (relative to conventional uranium oxide assemblies),which may have advantages with respect to pumping coolant through theassembly 10.

As shown in FIG. 4 , the element 20 is axially elongated. In theillustrated embodiment, each element 20 is a full-length element andextends the entire way from lower tie plate 70 at or near the bottom ofthe assembly 10 to the upper tie plate 80 at or near the top of theassembly 10. According to various embodiments and reactor designs, thismay result in elements 20 that are anywhere from 1 meter long (forcompact reactors) to over 4 meters long. Thus, for typical reactors, theelements 20 may be between 1 and 5 meters long. However, the elements 20may be lengthened or shortened to accommodate any other sized reactorwithout deviating from the scope of the present invention.

While the illustrated elements 20 are themselves full length, theelements 20 may alternatively be segmented, such that the multiplesegments together make a full length element. For example, 4 individual1 meter element segments 20 may be aligned end to end to effectivelycreate the full-length element. Additional tie plates 70, 80 may beprovided at the intersections between segments to maintain the axialspacing and arrangement of the segments.

According to one or more embodiments, the fuel kernel 100 comprises acombination of a refractory metal/alloy and fuel material. Therefractory metal/alloy may comprise a zirconium alloy. The fuel materialmay comprise low enriched uranium (e.g., U235, U233), plutonium, orthorium combined with low enriched uranium as defined below and/orplutonium. As used herein, “low enriched uranium” means that the wholefuel material contains less than 20% by weight fissile material (e.g.,uranium-235 or uranium-233). According to various embodiments, theuranium fuel material is enriched to between 1% and 20%, 5% and 20%, 10%and 20%, and/or 15% and 20% by weight of uranium-235. According to oneor more embodiments, the fuel material comprises 19.7% enricheduranium-235.

According to various embodiments, the fuel material may comprise a3-10%, 10-40%, 15-35%, and/or 20-30% volume fraction of the fuel kernel100. According to various embodiments, the refractory metal may comprisea 60-99%, 60-97%, 70-97%, 6090%, 65-85%, and/or 70-80% volume fractionof the fuel kernel 100. According to one or more embodiments, volumefractions within one or more of these ranges provide an alloy withbeneficial properties as defined by the material phase diagram for thespecified alloy composition. The fuel kernel 100 may comprise a Zr—Ualloy that is a high-alloy fuel (i.e., relatively high concentration ofthe alloy constituent relative to the uranium constituent) comprised ofeither δ-phase UZr₂, or a combination of δ-phase UZr₂ and α-phase Zr.According to one or more embodiments, the δ-phase of the U—Zr binaryalloy system may range from a zirconium composition of approximately65-81 volume percent (approximately 63 to 80 atom percent) of the fuelkernel 100. One or more of these embodiments have been found to resultin low volumetric, irradiation-induced swelling of the fuel element 20.According to one or more such embodiments, fission gases are entrainedwithin the metal kernel 100 itself, such that one or more embodiments ofthe fuel element 20 can omit a conventional gas gap from the fuelelement 20. According to one or more embodiments, such swelling may besignificantly less than would occur if low alloy (α-phase only)compositions were used (e.g., at least 10%, 20%, 30%, 50%, 75%, 100%,200%, 300%, 500%, 1000%, 1200%, 1500%, or greater reduction in volumepercent swelling per atom percent burnup than if a low alloy α-phaseU-10Zr fuel was used). According to one or more embodiments of thepresent invention, irradiation-induced swelling of the fuel element 20or kernel 100 thereof may be less than 20, 15, 10, 5, 4, 3, and/or 2volume percent per atom percent burnup. According to one or moreembodiments, swelling is expected to be around one volume percent peratom percent burnup.

According to one or more alternative embodiments of the presentinvention, the fuel kernel is replaced with a plutonium-zirconium binaryalloy with the same or similar volume percentages as with theabove-discussed U—Zr fuel kernels 100, or with different volumepercentages than with the above-discussed U—Zr fuel kernels 100. Forexample, the plutonium fraction in the kernel 100 may be substantiallyless than a corresponding uranium fraction in a correspondinguranium-based kernel 100 because plutonium typically has about 60-70%weight fraction of fissile isotopes, while LEU uranium has 20% or lessweight fraction of fissile U-235 isotopes. According to variousembodiments, the plutonium volume fraction in the kernel 100 may be lessthan 15%, less than 10%, and/or less than 5%, with the volume fractionof the refractory metal being adjusted accordingly.

The use of a high-alloy kernel 100 according to one or more embodimentsof the present invention may also result in the advantageous retentionof fission gases during irradiation. Oxide fuels and low-alloy metalfuels typically exhibit significant fission gas release that istypically accommodated by the fuel design, usually with a plenum withinthe fuel rod to contain released fission gases. The fuel kernel 100according to one or more embodiments of the present invention, incontrast, does not release fission gases. This is in part due to the lowoperating temperature of the fuel kernel 100 and the fact that fissiongas atoms (specifically Xe and Kr) behave like solid fission products.Fission gas bubble formation and migration along grain boundaries to theexterior of the fuel kernel 100 does not occur according to one or moreembodiments. At sufficiently high temperatures according to one or moreembodiments, small (a few micron diameter) fission gas bubbles may form.However, these bubbles remain isolated within the fuel kernel 100 and donot form an interconnected network that would facilitate fission gasrelease, according to one or more embodiments of the present invention.The metallurgical bond between the fuel kernel 100 and cladding 120 mayprovide an additional barrier to fission gas release.

According to various embodiments, the fuel kernel 100 (or the cladding120 or other suitable part of the fuel element 20) of one or more of thefuel elements 20 can be alloyed with a burnable poison such asgadolinium, boron, erbium or other suitable neutron absorbing materialto form an integral burnable poison fuel element. Different fuelelements 20 within a fuel assembly 10 may utilize different burnablepoisons and/or different amounts of burnable poison. For example, someof fuel elements 20 of a fuel assembly 10 (e.g., less than 75%, lessthan 50%, less than 20%, 1-15%, 1-12%, 2-12%, etc.) may include kernels100 with 25, 20, and/or 15 weight percent or less Gd (e.g., 1-25 weightpercent, 1-15 weight percent, 5-15 weight percent, etc.). Other fuelelements 20 of the fuel assembly 10 (e.g., 1095%, 10-50%, 20-50%, agreater number of the fuel elements 20 than the fuel elements 20 thatutilize Gd) may include kernels 100 with 10 or 5 weight percent or lessEr (e.g., 0.1-10.0 weight percent, 0.1 to 5.0 weight percent etc.).

According to various embodiments, the burnable poison displaces the fuelmaterial (rather than the refractory metal) relative to fuel elements 20that do not include burnable poison in their kernels 100. For example,according to one embodiment of a fuel element 20 whose kernel 100 wouldotherwise include 65 volume percent zirconium and 35 volume percenturanium in the absence of a poison, the fuel element 20 includes akernel 100 that is 16.5 volume percent Gd, 65 volume percent zirconium,and 18.5 volume percent uranium. According to one or more otherembodiments, the burnable poison instead displaces the refractory metal,rather than the fuel material. According to one or more otherembodiments, the burnable poison in the fuel kernel 100 displaces therefractory metal and the fuel material proportionally. Consequently,according to various of these embodiments, the burnable poison withinthe fuel kernel 100 may be disposed in the δ-phase of UZr₂ or α-phase ofZr such that the presence of the burnable poison does not change thephase of the UZr₂ alloy or Zr alloy in which the burnable poison isdisposed.

Fuel elements 20 with a kernel 100 with a burnable poison may make up aportion (e.g., 0-100%, 1-99%, 1-50%, etc.) of the fuel elements 20 ofone or more fuel assemblies 10 used in a reactor core. For example, fuelelements 20 with burnable poison may be positioned in strategiclocations within the fuel assembly lattice of the assembly 10 that alsoincludes fuel elements 20 without burnable poison to provide powerdistribution control and to reduce soluble boron concentrations early inthe operating cycle. Similarly, select fuel assemblies 10 that includefuel elements 20 with burnable poison may be positioned in strategiclocations within the reactor core relative to assemblies 10 that do notinclude fuel elements 20 with burnable poison to provide powerdistribution control and to reduce soluble boron concentrations early inthe operating cycle. The use of such integral burnable absorbers mayfacilitate the design of extended operating cycles.

Alternatively and/or additionally, separate non-fuel bearing burnablepoison rods may be included in the fuel assembly 10 (e.g., adjacent tofuel elements 20, in place of one or more fuel elements 20, insertedinto guide tubes in fuel assemblies 10 that do not receive control rods,etc.). In one or more embodiments, such non-fuel burnable poison rodscan be designed into a spider assembly similar to that which is used inthe Babcock and Wilcox or Westinghouse designed reactors (referred to asburnable poison rod assemblies (BPRA)). These then may be inserted intothe control rod guide tubes and locked into select fuel assemblies 10where there are no control banks for the initial cycle of operation forreactivity control. When the burnable poison cluster is used it may beremoved when the fuel assembly is relocated for the next fuel cycle.According to an alternative embodiment in which the separate non-fuelbearing burnable poison rods are positioned in place of one or more fuelelements 20, the non-fuel burnable poison rods remain in the fuelassembly 10 and are discharged along with other fuel elements 20 whenthe fuel assembly 10 reaches its usable life.

The fuel elements 20 are manufactured via powder-metallurgyco-extrusion. Typically, the powdered refractory metal and powderedmetal fuel material (as well as the powdered burnable poison, ifincluded in the kernel 100) for the fuel kernel 100 are mixed, thedisplacer 110 blank is positioned within the powder mixture, and thenthe combination of powder and displacer 110 is pressed and sintered intofuel core stock/billet (e.g., in a mold that is heated to varyingextents over various time periods so as to sinter the mixture). Thedisplacer 110 blank may have the same or similar cross-sectional shapeas the ultimately formed displacer 110. Alternatively, the displacer 110blank may have a shape that is designed to deform into the intendedcross-sectional shape of the displacer 110 upon extrusion. The fuel corestock (including the displacer 110 and the sintered fuel kernel 100material) is inserted into a hollow cladding 120 tube that has a sealedtube base and an opening on the other end. The opening on the other endis then sealed by an end plug made of the same material as the claddingto form a billet. The billet may be cylindrically shaped, or may have ashape that more closely resembles the ultimate cross-sectional shape ofthe element 20, for example, as shown in FIGS. 5 and 9 . The billet isthen co-extruded under temperature and pressure through a die set tocreate the element 20, including the finally shaped kernel 100, cladding110, and displacer 120. According to various embodiments that utilize anon-cylindrical displacer 110, the billet may be properly orientedrelative to the extrusion press die so that corners of the displacer 110align with the lobes 20 b of the fuel element 20. The extrusion processmay be done by either direct extrusion (i.e., moving the billet througha stationary die) or indirect extrusion (i.e., moving the die toward astationary billet). The process results in the cladding 120 beingmetallurgically bonded to the fuel kernel 100, which reduces the risk ofdelamination of the cladding 120 from the fuel kernel 100. The tube andend plug of the cladding 120 metallurgically bond to each other to sealthe fuel kernel 100 within the cladding 120. The high melting points ofrefractory metals used in the fuel elements 10 tend to make powdermetallurgy the method of choice for fabricating components from thesemetals.

According to one or more alternative embodiments, the fuel core stock ofthe fuel elements 20 may be manufactured via casting instead ofsintering. Powdered or monolithic refractory metal and powdered ormonolithic fuel material (as well as the powdered burnable poison, ifincluded in the kernel 100) may be mixed, melted, and cast into a mold.The mold may create a displacer-blank-shaped void in the cast kernel 100such that the displacer 110 blank may be inserted after the kernel 100is cast, in the same manner that the cladding 120 is added to form thebillet to be extruded. The remaining steps for manufacturing the fuelelements 20 may remain the same as or similar to the above-discussembodiment that utilizes sintering instead of casting. Subsequentextrusion results in metallurgical bonding between the displacer 110 andkernel 100, as well as between the kernel 100 and cladding 120.

According to one or more alternative embodiments, the fuel elements 20are manufactured using powdered ceramic fuel material instead ofpowdered metal fuel material. The remaining manufacturing steps may bethe same as discussed above with respect to the embodiments usingpowdered metal fuel material. In various metal fuel embodiments andceramic fuel embodiments, the manufacturing process may result in a fuelkernel 100 comprising fuel material disposed in a matrix of metalnon-fuel material. In one or more of the metal fuel embodiments, theresulting fuel kernel 100 comprises a metal fuel alloy kernel comprisingan alloy of the metal fuel material and the matrix of metal non-fuelmaterial (e.g., a uranium-zirconium alloy). In one or more of theceramic fuel embodiments, the kernel 100 comprises ceramic fuel materialdisposed in (e.g., interspersed throughout) the matrix of metal non-fuelmaterial. According to various embodiments, the ceramic fuel materialused in the manufacturing process may comprise powdered uranium orplutonium oxide, powdered uranium or plutonium nitride, powdered uraniumor plutonium carbide, powdered uranium or plutonium hydride, or acombination thereof. In contrast with conventional UO₂ fuel elements inwhich UO₂ pellets are disposed in a tube, the manufacturing processaccording to one or more embodiments of the present invention results inceramic fuel being disposed in a solid matrix of non-fuel material(e.g., a zirconium matrix).

As shown in FIG. 4 , the axial coiling pitch of the spiral ribs 130 isselected according to the condition of placing the axes of adjacent fuelelements 10 with a spacing equal to the width across corners in thecross section of a fuel element and may be 5% to 20% of the fuel element20 length. According to one embodiment, the pitch (i.e., the axiallength over which a lobe/rib makes a complete rotation) is about 21.5cm, while the full active length of the element 20 is about 420 cm. Asshown in FIG. 3 , stability of the vertical arrangement of the fuelelements 10 is provided: at the bottom—by the lower tie plate 70; at thetop—by the upper tie plate 80; and relative to the height of the core—bythe shroud 30. As shown in FIG. 1 , the fuel elements 10 have acircumferential orientation such that the lobed profiles of any twoadjacent fuel elements 10 have a common plane of symmetry which passesthrough the axes of the two adjacent fuel elements 10 in at least onecross section of the fuel element bundle.

As shown in FIG. 1 , the helical twist of the fuel elements 20 incombination with their orientation ensures that there exists one or moreself-spacing planes. As shown in FIG. 1 , in such self spacing planes,the ribs of adjacent elements 20 contact each other to ensure properspacing between such elements 20. Thus, the center-to-center spacing ofelements 20 will be about the same as the corner-to-corner width of eachelement 20 (12.6 mm in the element illustrated in FIG. 5 ). Depending onthe number of lobes 20 b in each fuel element 20 and the relativegeometrical arrangement of the fuel elements 20, all adjacent fuelelements 20 or only a portion of the adjacent fuel elements 20 willcontact each other. For example, in the illustrated four-lobedembodiment, each fuel element 20 contacts all four adjacent fuelelements 20 at each self-spacing plane. However, in a three-lobed fuelelement embodiment in which the fuel elements are arranged in ahexagonal pattern, each fuel element will only contact three of the sixadjacent fuel elements in a given self-spacing plane. The three-lobedfuel element will contact the other three adjacent fuel elements in thenext axially-spaced self-spacing plane (i.e., ⅙ of a turn offset fromthe previous self-spacing plane).

In an n-lobed element 20 in which n fuel elements are adjacent to aparticular fuel element 20, a self-spacing plane will exist every 1/nhelical turn (e.g., every ¼ helical turn for a four-lobed element 20arranged in a square pattern such that four other fuel elements 20 areadjacent to the fuel element 20; every ⅓ helical turn for a three-lobedelement in which three fuel elements are adjacent to the fuel element(i.e., every 120 degrees around the perimeter of the fuel element)). Thepitch of the helix may be modified to create greater or fewerself-spacing planes over the axial length of the fuel elements 20.According to one embodiment, each four-lobed fuel element 20 includesmultiple twists such that there are multiple self-spacing planes overthe axial length of the bundle of fuel elements 20.

In the illustrated embodiment, all of the elements 20 twist in the samedirection. However, according to an alternative embodiment, adjacentelements 20 may twist in opposite directions without deviating from thescope of the present invention.

The formula for the number of self-spacing planes along the fuel rodlength is as follows:

N=n*L/h, where:

-   -   L—Fuel rod length    -   n—Number of lobes (ribs) and the number of fuel elements        adjacent to a fuel element    -   h—Helical twist pitch        The formula is slightly different if the number of lobes and the        number of fuel elements adjacent to a fuel element are not the        same.

As a result of such self-spacing, the fuel assembly 10 may omit spacergrids that may otherwise have been necessary to assure proper elementspacing along the length of the assembly 10. By eliminating spacergrids, coolant may more freely flow through the assembly 10, whichadvantageously increases the heat transfer from the elements 20 to thecoolant. However, according to alternative embodiments of the presentinvention, the assembly 10 may include spacer grid(s) without deviatingfrom the scope of the present invention.

As shown in FIG. 3 , the shroud 30 forms a tubular shell that extendsaxially along the entire length of the fuel elements 20 and surroundsthe elements 20. However, according to an alternative embodiment of thepresent invention, the shroud 30 may comprise axially-spaced bands, eachof which surrounds the fuel elements 20. One or more such bands may beaxially aligned with the self-spacing planes. Axially extending cornersupports may extend between such axially spaced bands to support thebands, maintain the bands' alignment, and strengthen the assembly.Alternatively and/or additionally, holes may be cut into the otherwisetubular/polygonal shroud 30 in places where the shroud 30 is not neededor desired for support. Use of a full shroud 30 may facilitate greatercontrol of the separate coolant flows through each individual fuelassembly 10. Conversely, the use of bands or a shroud with holes mayfacilitate better coolant mixing between adjacent fuel assemblies 10,which may advantageously reduce coolant temperature gradients betweenadjacent fuel assemblies 10.

As shown in FIG. 1 , the cross-sectional perimeter of the shroud 30 hasa shape that accommodates the reactor in which the assembly 10 is used.In reactors such as the AP-1000 that utilize square fuel assemblies, theshroud has a square cross-section. However, the shroud 30 mayalternatively take any suitable shape depending on the reactor in whichit is used (e.g., a hexagonal shape for use in a VVER reactor (e.g., asshown in FIG. 1 of U.S. Patent Application Publication No. 2009/0252278A1).

The guide tubes 40 provide for the insertion of control absorberelements based on boron carbide (B₄C), silver indium cadmium (Ag, In,Cd), dysprosium titanate (Dy₂0₃.TiO₂) or other suitable alloys ormaterials used for reactivity control (not shown) and burnable absorberelements based on boron carbide, gadolinium oxide (Gd₂O₃) or othersuitable materials (not shown) and are placed in the upper nozzle 50with the capability of elastic axial displacement. The guide tubes 40may comprise a zirconium alloy. For example, the guide tube 40arrangement shown in FIG. 1 is in an arrangement used in the AP-1000reactor (e.g., 24 guide tubes arranged in two annular rows at thepositions shown in the 17×17 grid).

The shape, size, and features of the frame 25 depend on the specificreactor core for which the assembly 10 is to be used. Thus, one ofordinary skill in the art would understand how to make appropriatelyshaped and sized frame for the fuel assembly 10. For example, the frame25 may be shaped and configured to fit into a reactor core of aconventional nuclear power plant in place of a conventional uraniumoxide or mixed oxide fuel assembly for that plant's reactor core. Thenuclear power plant may comprise a reactor core design that was inactual use before 2010 (e.g., 2, 3 or 4-loop PWRs; BWR-4).Alternatively, the nuclear power plant may be of an entirely new designthat is specifically tailored for use with the fuel assembly 10.

As explained above, the illustrated fuel assembly 10 is designed for usein an AP-1000 or EPR reactor. The assembly includes a 17×17 array offuel elements 20, 24 of which are replaced with guide tubes 40 asexplained above for a total of 265 fuel elements 20 in EPR or 264 fuelelements 20 in AP-1000 (in the AP-1000, in addition to the 24 fuelelements being replaced with the guide tubes, a central fuel element isalso replaced with an instrumented tube).

The elements 20 preferably provide 100% of the overall fissile materialof the fuel assembly 10. Alternatively, some of the fissile material ofthe assembly 10 may be provided via fuel elements other than theelements 20 (e.g., non-lobed fuel elements, uranium oxide elements,elements having fuel ratios and/or enrichments that differ from theelements 20). According to various such alternative embodiments, thefuel elements 20 provide at least 50%, 60%, 70%, 75%, 80%, 85%, 90%,and/or 95% by volume of the overall fissile material of the fuelassembly 10.

Use of the metal fuel elements 20 according to one or more embodimentsof the present invention facilitate various advantages over the uraniumoxide or mixed oxide fuel conventionally used in light water nuclearreactors (LWR) (including boiling water reactors and pressurized waterreactors) such as the Westinghouse-designed AP-1000, AREVA-designed EPRreactors, or GE-designed ABWR. For example, according to one or moreembodiments, the power rating for an LWR operating on standard uraniumoxide or mixed oxide fuel could be increased by up to about 30% bysubstituting the all-metal fuel elements 20 and/or fuel assembly 10 forstandard uranium oxide fuel and fuel assemblies currently used inexisting types of LWRs or new types of LWRs that have been proposed.

One of the key constraints for increasing power rating of LWRs operatingon standard uranium oxide fuel has been the small surface area ofcylindrical fuel elements that such fuel utilizes. A cylindrical fuelelement has the lowest surface area to volume ratio for any type of fuelelement cross-section profile. Another major constraint for standarduranium oxide fuel has been a relatively low burnup that such fuelelements could possibly reach while still meeting acceptable fuelperformance criteria. As a result, these factors associated withstandard uranium oxide or mixed oxide fuel significantly limit thedegree to which existing reactor power rating could be increased.

One or more embodiments of the all-metal fuel elements 20 overcome theabove limitations. For example, as explained above, the lack of spacergrids may reduce hydraulic resistance, and therefore increase coolantflow and heat flux from the elements 20 to the primary coolant. Thehelical twist of the fuel elements 20 may increase coolant intermixingand turbulence, which may also increase heat flux from the elements 20to the coolant.

Preliminary neutronic and thermal-hydraulic analyses have shown thefollowing according to one or more embodiments of the present invention:

-   -   The thermal power rating of an LWR reactor could be increased by        up to 30.7% or more (e.g., the thermal power rating of an EPR        reactor could be increased from 4.59 GWth to 6.0 GWth).    -   With a uranium volume fraction of 25% in the uranium-zirconium        mixture and uranium-235 enrichment of 19.7%, an EPR reactor core        with a four-lobe metallic fuel element 20 configuration could        operate for about 500-520 effective full power days (EFPDs) at        the increased thermal power rating of 6.0 GWth if 72 fuel        assemblies were replaced per batch (once every 18 months) or        540-560 EFPDs if 80 fuel assemblies were replaced per batch        (once every 18 months).    -   Due to the increased surface area in the multi-lobe fuel        element, even at the increased power rating of 6.0 GWth, the        average surface heat flux of the multi-lobe fuel element is        shown to be 4-5% lower than that for cylindrical uranium oxide        fuel elements operating at the thermal power rating of 4.59        GWth. This could provide an increased safety margin with respect        to critical heat flux (e.g., increased departure from nucleate        boiling margin in PWRs or maximum fraction limiting critical        power ratio in BWRs). Further, this could allow a possibility of        using 12 fuel elements per assembly with burnable poisons.        Burnable poisons could be used to remove excess reactivity at        the beginning of cycle or to increase the Doppler Effect during        the heat-up of the core.    -   Thus, the fuel assemblies 10 may provide greater thermal power        output at a lower fuel operating temperature than conventional        uranium oxide or mixed oxide fuel assemblies.

To utilize the increased power output of the assembly 10, conventionalpower plants could be upgraded (e.g., larger and/or additional coolantpumps, steam generators, heat exchangers, pressurizers, turbines).Indeed, according to one or more embodiments, the upgrade could provide30-40% more electricity from an existing reactor. Such a possibility mayavoid the need to build a complete second reactor. The modification costmay quickly pay for itself via increased electrical output.Alternatively, new power plants could be constructed to include adequatefeatures to handle and utilize the higher thermal output of theassemblies 10.

Further, one or more embodiments of the present invention could allow anLWR to operate at the same power rating as with standard uranium oxideor mixed oxide fuel using existing reactor systems without any majorreactor modifications. For example, according to one embodiment:

-   -   An EPR would have the same power output as if conventional        uranium-oxide fuel were used: 4.59 GWt;    -   With a uranium volume fraction of 25% in the uranium-zirconium        mixture and uranium-235 enrichment of approximately 15%, an EPR        reactor core with a four-lobe metallic fuel element 20        configuration could operate for about 500-520 effective full        power days (EFPDs) if 72 fuel assemblies were replaced per batch        or 540-560 EFPDs if 80 fuel assemblies were replaced per batch.    -   The average surface heat flux for the elements 20 is reduced by        approximately 30% compared to that for cylindrical rods with        conventional uranium oxide fuel (e.g., 39.94 v. 57.34 W/cm²).        Because the temperature rise of the coolant through the assembly        10 (e.g., the difference between the inlet and outlet        temperature) and the coolant flow rate through the assembly 10        remain approximately the same relative to conventional fuel        assemblies, the reduced average surface heat flux results in a        corresponding reduction in the fuel rod surface temperature that        contributes to increased safety margins with respect to critical        heat flux (e.g., increased departure from nucleate boiling        margin in PWRs or maximum fraction limiting critical power ratio        in BWRs).

Additionally and/or alternatively, fuel assemblies 10 according to oneor more embodiments of the present invention can be phased/laddered intoa reactor core in place of conventional fuel assemblies. During thetransition period, fuel assemblies 10 having comparablefissile/neutronic/thermal outputs as conventional fuel assemblies cangradually replace such conventional fuel assemblies over sequential fuelchanges without changing the operating parameters of the power plant.Thus, fuel assemblies 10 can be retrofitted into an existing core thatmay be important during a transition period (i.e., start with a partialcore with fuel assemblies 10 and gradually transition to a full core offuel assemblies 10).

Moreover, the fissile loading of assemblies 10 can be tailored to theparticular transition desired by a plant operator. For example, thefissile loading can be increased appropriately so as to increase thethermal output of the reactor by anywhere from 0% to 30% or more higher,relative to the use of conventional fuel assemblies that the assemblies10 replace. Consequently, the power plant operator can chose thespecific power uprate desired, based on the existing plantinfrastructure or the capabilities of the power plant at various timesduring upgrades.

One or more embodiments of the fuel assemblies 10 and fuel elements 20may be used in fast reactors (as opposed to light water reactors)without deviating from the scope of the present invention. In fastreactors, the non-fuel metal of the fuel kernel 100 is preferably arefractory metal, for example a molybdenum alloy (e.g., pure molybdenumor a combination of molybdenum and other metals), and the cladding 120is preferably stainless steel (which includes any alloy variationthereof) or other material suitable for use with coolant in suchreactors (e.g., sodium). Such fuel elements 20 may be manufactured viathe above-discussed co-extrusion process or may be manufactured by anyother suitable method (e.g., vacuum melt).

As shown in FIGS. 7A, 7B, and 8 , fuel assemblies 510 accordingly to oneor more embodiments of the present invention may be used in apressurized heavy water reactor 500 (see FIG. 8 ) such as a CANDUreactor.

As shown in FIGS. 7A and 7B, the fuel assembly 510 comprises a pluralityof fuel elements 20 mounted to a frame 520. The frame 520 comprises twoend plates 520 a, 520 b that mount to opposite axial ends of the fuelelements 20 (e.g., via welding, interference fits, any of the varioustypes of attachment methods described above for attaching the elements20 to the lower tie plate 70). The elements 20 used in the fuel assembly510 are typically much shorter than the elements 20 used in the assembly10. According to various embodiments and reactors 500, the elements 20and assemblies 510 used in the reactor 500 may be about 18 inches long.

The elements 20 may be positioned relative to each other in the assembly510 so that self-spacing planes maintain spacing between the elements 20in the manner described above with respect to the assembly 10.Alternatively, the elements 20 of the assembly 510 may be so spaced fromeach other that adjacent elements 20 never touch each other, and insteadrely entirely on the frame 520 to maintain element 20 spacing.Additionally, spacers may be attached to the elements 20 or their ribsat various positions along the axial length of the elements 20 tocontact adjacent elements 20 and help maintain element spacing 20 (e.g.,in a manner similar to how spacers are used on conventional fuel rods ofconventional fuel assemblies for pressurized heavy water reactors tohelp maintain rod spacing).

As shown in FIG. 8 , the assemblies 510 are fed into calandria tubes 500a of the reactor 500 (sometimes referred to in the art as a calandria500). The reactor 500 uses heavy water 500 b as a moderator and primarycoolant. The primary coolant 500 b circulates horizontally through thetubes 500 a and then to a heat exchanger where heat is transferred to asecondary coolant loop that is typically used to generate electricityvia turbines. Fuel assembly loading mechanisms (not shown) are used toload fuel assemblies 510 into one side of the calandria tubes 500 a andpush spent assemblies 510 out of the opposite side of the tubes 500 a,typically while the reactor 500 is operating.

The fuel assemblies 510 may be designed to be a direct substitute forconventional fuel assemblies (also known as fuel bundles in the art) forexisting, conventional pressurized heavy water reactors (e.g., CANDUreactors). In such an embodiment, the assemblies 510 are fed into thereactor 500 in place of the conventional assemblies/bundles. Such fuelassemblies 510 may be designed to have neutronic/thermal propertiessimilar to the conventional assemblies being replaced. Alternatively,the fuel assemblies 510 may be designed to provide a thermal poweruprate. In such uprate embodiments, new or upgraded reactors 500 can bedesigned to accommodate the higher thermal output.

According to various embodiments of the present invention, the fuelassembly 10 is designed to replace a conventional fuel assembly of aconventional nuclear reactor. For example, the fuel assembly 10illustrated in FIG. 1 is specifically designed to replace a conventionalfuel assembly that utilizes a 17×17 array of UO₂ fuel rods. If the guidetubes 40 of the assembly 10 are left in the exact same position as theywould be for use with a conventional fuel assembly, and if all of thefuel elements 20 are the same size, then the pitch between fuelelements/rods remains unchanged between the conventional UO₂ fuelassembly and one or more embodiments of the fuel assembly 10 (e.g., 12.6mm pitch). In other words, the longitudinal axes of the fuel elements 20may be disposed in the same locations as the longitudinal axes ofconventional UO₂ fuel rods would be in a comparable conventional fuelassembly. According to various embodiments, the fuel elements 20 mayhave a larger circumscribed diameter than the comparable UO₂ fuel rods(e.g., 12.6 mm as compared to an outer diameter of 9.5 mm for a typicalUO₂ fuel rod). As a result, in the self-aligning plane illustrated inFIG. 1 , the cross-sectional length and width of the space occupied bythe fuel elements 20 may be slightly larger than that occupied byconventional UO₂ fuel rods in a conventional fuel assembly (e.g., 214.2mm for the fuel assembly 10 (i.e., 17 fuel elements 20×12.6 mmcircumscribed diameter per fuel element), as opposed to 211.1 mm for aconventional UO₂ fuel assembly that includes a 17×17 array of 9.5 mm UO₂fuel rods separated from each other by a 12.6 mm pitch). In conventionalUO₂ fuel assemblies, a spacer grid surrounds the fuel rods, andincreases the overall cross-sectional envelope of the conventional fuelassembly to 214 mm×214 mm. In the fuel assembly 10, the shroud 30similarly increases the cross-sectional envelope of the fuel assembly10. The shroud 30 may be any suitable thickness (e.g., 0.5 mm or 1.0 mmthick). In an embodiment that utilizes a 1.0 mm thick shroud 30, theoverall cross-sectional envelope of an embodiment of the fuel assembly10 may be 216.2 mm×216.2 mm (e.g., the 214 mm occupied by the 1712.6 mmdiameter fuel elements 20 plus twice the 1.0 mm thickness of the shroud30). As a result, according to one or more embodiments of the presentinvention, the fuel assembly 10 may be slightly larger (e.g., 216.2mm×216.2 mm) than a typical UO₂ fuel assembly (214 mm×214 mm). Thelarger size may impair the ability of the assembly 10 to properly fitinto the fuel assembly positions of one or more conventional reactors,which were designed for use with conventional UO₂ fuel assemblies. Toaccommodate this size change, according to one or more embodiments ofthe present invention, a new reactor may be designed and built toaccommodate the larger size of the fuel assemblies 10.

According to an alternative embodiment of the present invention, thecircumscribed diameter of all of the fuel elements 20 may be reducedslightly so as to reduce the overall cross-sectional size of the fuelassembly 10. For example, the circumscribed diameter of each fuelelement 20 may be reduced by 0.13 mm to 12.47 mm, so that the overallcross-sectional space occupied by the fuel assembly 10 remainscomparable to a conventional 214 mm by 214 mm fuel assembly (e.g.,1712.47 mm diameter fuel elements 20 plus two 1.0 mm thickness of theshroud, which totals about 214 mm). Such a reduction in the size of the17 by 17 array will slightly change the positions of the guide tubes 40in the fuel assembly 10 relative to the guide tube positions in aconventional fuel assembly. To accommodate this slight position changein the tube 40 positions, the positions of the corresponding control rodarray and control rod drive mechanisms in the reactor may be similarlyshifted to accommodate the repositioned guide tubes 40. Alternatively,if sufficient clearances and tolerances are provided for the controlrods in a conventional reactor, conventionally positioned control rodsmay adequately fit into the slightly shifted tubes 40 of the fuelassembly 10.

Alternatively, the diameter of the peripheral fuel elements 20 may bereduced slightly so that the overall assembly 10 fits into aconventional reactor designed for conventional fuel assemblies. Forexample, the circumscribed diameter of the outer row of fuel elements 20may be reduced by 1.1 mm such that the total size of the fuel assemblyis 214 mm×214 mm (e.g., 1512.6 mm fuel elements 20 plus 211.5 mm fuelelements 20 plus 21.0 mm thicknesses of the shroud 30). Alternatively,the circumscribed diameter of the outer two rows of fuel elements 20 maybe reduced by 0.55 mm each such that the total size of the fuel assemblyremains 214 mm×214 mm (e.g., 1312.6 mm fuel elements 20 plus 412.05 mmfuel assemblies plus 21.0 mm thicknesses of the shroud 30). In eachembodiment, the pitch and position of the central 13×13 array of fuelelements 20 and guide tubes 40 remains unaltered such that the guidetubes 40 align with the control rod array and control rod drivemechanisms in a conventional reactor.

FIG. 10 illustrates a fuel assembly 610 according to an alternativeembodiment of the present invention. According to various embodiments,the fuel assembly 610 is designed to replace a conventional UO₂ fuelassembly in a conventional reactor while maintaining the control rodpositioning of reactors designed for use with various conventional UO₂fuel assemblies. The fuel assembly 610 is generally similar to the fuelassembly 10, which is described above and illustrated in FIG. 1 , butincludes several differences that help the assembly 610 to better fitinto one or more existing reactor types (e.g., reactors usingWestinghouse's fuel assembly design that utilizes a 17 by 17 array ofUO₂ rods) without modifying the control rod positions or control roddrive mechanisms.

As shown in FIG. 10 , the fuel assembly includes a 17 by 17 array ofspaces. The central 15 by 15 array is occupied by 200 fuel elements 20and 25 guide tubes 40, as described above with respect to the similarfuel assembly 10 illustrated in FIG. 1 . Depending on the specificreactor design, the central guide tube 40 may be replaced by anadditional fuel element 20 if the reactor design does not utilize acentral tube 40 (i.e., 201 fuel elements 20 and 24 guide tubes 40). Theguide tube 40 positions correspond to the guide tube positions used inreactors designed to use conventional UO₂ fuel assemblies.

The peripheral positions (i.e., the positions disposed laterally outwardfrom the fuel elements 20) of the 17 by 17 array/pattern of the fuelassembly 610 are occupied by 64 UO₂ fuel elements/rods 650. As is knownin the art, the fuel rods 650 may comprise standard UO₂ pelletized fueldisposed in a hollow rod. The UO₂ pelletized fuel may be enriched withU-235 by less than 20%, less than 15%, less than 10%, and/or less than5%. The rods 650 may have a slightly smaller diameter (e.g., 9.50 mm)than the circumscribed diameter of the fuel elements 20, which slightlyreduces the overall cross-sectional dimensions of the fuel assembly 610so that the assembly 610 better fits into the space allocated for aconventional UO₂ fuel assembly.

In the illustrated embodiment, the fuel rods/elements 650 comprise UO₂pelletized fuel. However, the fuel rods/elements 650 may alternativelyutilize any other suitable combination of one or more fissile and/orfertile materials (e.g., thorium, plutonium, uranium-235, uranium-233,any combinations thereof). Such fuel rods/elements 650 may comprisemetal and/or oxide fuel.

According to one or more alternative embodiments, the fuel rods 650 mayoccupy less than all of the 64 peripheral positions. For example, thefuel rods 650 may occupy the top row and left column of the periphery,while the bottom row and right column of the periphery may be occupiedby fuel elements 20. Alternatively, the fuel rods 650 may occupy anyother two sides of the periphery of the fuel assembly. The shroud 630may be modified so as to enclose the additional fuel elements 20 in theperiphery of the fuel assembly. Such modified fuel assemblies may bepositioned adjacent each other such that a row/column of peripheral fuelelements 650 in one assembly is always adjacent to a row/column of fuelelements 20 in the adjacent fuel assembly. As a result, additional spacefor the fuel assemblies is provided by the fact that the interfacebetween adjacent assemblies is shifted slightly toward the assembly thatincludes fuel elements 650 in the peripheral, interface side. Such amodification may provide for the use of a greater number of higher heatoutput fuel elements 20 than is provided by the fuel assemblies 610.

A shroud 630 surrounds the array of fuel elements 20 and separates theelements 20 from the elements 650. The nozzles 50, 60, shroud 630,coolant passages formed therebetween, relative pressure drops throughthe elements 20 and elements 650, and/or the increased pressure dropthrough the spacer grid 660 (discussed below) surrounding the elements650 may result in a higher coolant flow rate within the shroud 630 andpast the higher heat output fuel elements 20 than the flow rate outsideof the shroud 630 and past the relatively lower heat output fuel rods650. The passageways and/or orifices therein may be designed to optimizethe relative coolant flow rates past the elements 20, 650 based on theirrespective heat outputs and designed operating temperatures.

According to various embodiments, the moderator:fuel ratio for the fuelelements 20 of the fuel assembly 610 is less than or equal to 2.7, 2.6,2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, and/or 1.8. In the illustratedembodiment, the moderator:fuel ratio equals a ratio of (1) the totalarea within the shroud 630 available for coolant/moderator (e.g.,approximated by the total cross-sectional area within the shroud 630minus the total cross-sectional area taken up by the fuel elements 20(assuming the guide tubes 40 are filled with coolant)) to (2) the totalcross-sectional area of the kernels 100 of the fuel elements 20 withinthe shroud 630.

According to an alternative embodiment of the invention, the shroud 630may be replaced with one or more annular bands or may be provided withholes in the shroud 630, as explained above. The use of bands or holesin the shroud 630 may facilitate cross-mixing of coolant between thefuel elements 20 and the fuel elements 650.

As shown in FIG. 10 , the fuel elements 650 are disposed within anannular spacer grid 660 that is generally comparable to the outer partof a spacer grid used in a conventional UO₂ fuel assembly. The spacergrid 660 may rigidly connect to the shroud 630 (e.g., via welds, bolts,screws, or other fasteners). The spacer grid 660 is preferably sized soas to provide the same pitch between the fuel elements 650 and the fuelelements 20 as is provided between the central fuel elements 20 (e.g.,12.6 mm pitch between axes of all fuel elements 20, 650). To providesuch spacing, the fuel elements 650 may be disposed closer to the outerside of the spacer grid 660 than to the shroud 630 and inner side of thespacer grid 660. The fuel assembly 610 and spacer grid 660 are alsopreferably sized and positioned such that the same pitch is providedbetween fuel elements 650 of adjacent fuel assemblies (e.g., 12.6 mmpitch). However, the spacing between any of the fuel elements 20, 650may vary relative to the spacing between other fuel elements 20, 650without deviating from the scope of the present invention.

According to various embodiments, the fuel elements 20 provide at least60%, 65%, 70%, 75%, and/or 80% of a total volume of allfissile-material-containing fuel elements 20, 650 of the fuel assembly610. For example, according to one or more embodiments in which the fuelassembly 610 includes 201 fuel elements 20, each having across-sectional area of about 70 mm², and 64 fuel elements 650, eachhaving a 9.5 mm diameter, the fuel elements 20 provide about 75.6% of atotal volume of all fuel elements 20, 650 (201 fuel elements 20×70 mm²equals 14070 mm²; 64 fuel elements 650×π×(9.5/2)²=4534 mm²; fuel element20, 650 areas are essentially proportional to fuel element volumes;(14070 mm²/(14070 mm²+4534 mm²)=75.6%)).

The height of the fuel assembly 610 matches a height of a comparableconventional fuel assembly that the assembly 610 can replace (e.g., theheight of a standard fuel assembly for a Westinghouse or AREVA reactordesign).

The illustrated fuel assembly 610 may be used in a 17×17 PWR such as theWestinghouse 4-loop design, AP1000, or AREVA EPR. However, the design ofthe fuel assembly 610 may also be modified to accommodate a variety ofother reactor designs (e.g., reactor designs that utilize a hexagonalfuel assembly, in which case the outer periphery of the hexagon isoccupied by UO₂ rods, while the inner positions are occupied by fuelelements 20, or boiling water reactors, or small modular reactors).While particular dimensions are described with regard to particularembodiments, a variety of alternatively dimensioned fuel elements 20,650 and fuel assemblies 10 may be used in connection with a variety ofreactors or reactor types without deviating from the scope of thepresent invention.

Depending on the specific reactor design, additional rod positions of afuel assembly may be replaced with UO₂ rods. For example, while the fuelassembly 610 includes UO₂ rods only in the outer peripheral row, theassembly 610 could alternatively include UO₂ rods in the outer two rowswithout deviating from the scope of the present invention.

According to various embodiments, the portion of the fuel assembly 610that supports the fuel elements 650 is inseparable from the portion ofthe fuel assembly 610 that supports the fuel elements 20. According tovarious embodiments, the fuel elements 20 are not separable as a unitfrom the fuel elements 650 of the fuel assembly 610 (even thoughindividual fuel elements 20, 650 may be removed from the assembly 610,for example, based on individual fuel element failure). Similarly, thereis not a locking mechanism that selectively locks the fuel element 650portion of the fuel assembly to the fuel element 20 portion of the fuelassembly 610. According to various embodiments, the fuel elements 20 andfuel elements 650 of the fuel assembly 610 have the same designed lifecycle, such that the entire fuel assembly 610 is used within thereactor, and then removed as a single spent unit.

According to various embodiments, the increased heat output of the fuelelements 20 within the fuel assembly 610 can provide a power upraterelative to the conventional all UO₂ fuel rod assembly that the assembly610 replaces. According to various embodiments, the power uprate is atleast 5%, 10%, and/or 15%. The uprate may be between 1 and 30%, 5 and25%, and/or 10 and 20% according to various embodiments. According tovarious embodiments, the fuel assembly 610 provides at least an 18-monthfuel cycle, but may also facilitate moving to a 24+ or 36+ month fuelcycle. According to an embodiment of the fuel assembly 610, which usesfuel elements 20 having the example parameters discussed above withrespect to the element 20 shown in FIG. 10 , the assembly 17 provides a17% uprate relative to a conventional UO₂ fuel assembly under theoperating parameters identified in the below tables.

Operating Parameter for AREVA EPR Reactor Value Unit Reactor power 5.37GWt Fuel cycle length 18 months Reload batch size 1/3 core Enrichment ofFuel Element 20 <19.7 w/o Enrichment of UO₂ of the Rods 650 <5 w/oCoolant flow rate 117% rv * rv = reference value

Fuel Assembly Parameter Value Unit Fuel assembly design 17 × 17 Fuelassembly pitch 215 mm Fuel assembly envelope 214 mm Active fuel height4200 mm Number of fuel rods 265 Fuel element 20 pitch (i.e., axis toaxis spacing) 12.6 mm Average outer fuel element 20 diameter 12.6 mm(circumscribed diameter) Average minimum fuel element 20 diameter 10.44mm Moderator to fuel ratio, seed region (around 2.36 elements 20)Moderator to fuel ratio, blanket (around the fuel rods 1.9 650)

The fuel assemblies 10, 510, 610 are preferably thermodynamicallydesigned for and physically shaped for use in a land-based nuclear powerreactor 90, 500 (e.g., land-based LWRS (including BWRs and PWRs),land-based fast reactors, land-based heavy water reactors) that isdesigned to generate electricity and/or heat that is used for a purposeother than electricity (e.g., desalinization, chemical processing, steamgeneration, etc.). Such land-based nuclear power reactors 90 include,among others, VVER, AP-1000, EPR, APR-1400, ABWR, BWR-6, CANDU, BN-600,BN-800, Toshiba 4S, Monju, etc. However, according to alternativeembodiments of the present invention, the fuel assemblies 10, 510, 610may be designed for use in and used in marine-based nuclear reactors(e.g., ship or submarine power plants; floating power plants designed togenerate power (e.g., electricity) for onshore use) or other nuclearreactor applications.

The foregoing illustrated embodiments are provided to illustrate thestructural and functional principles of the present invention and arenot intended to be limiting. To the contrary, the principles of thepresent invention are intended to encompass any and all changes,alterations and/or substitutions within the spirit and scope of thefollowing claims.

What is claimed is:
 1. A method of manufacturing a fuel assembly for usein a core of a nuclear power reactor, the method comprising:manufacturing each of a plurality of elongated fuel elements by: mixingfuel material with metal non-fuel material, wherein the fuel materialcomprises fissile material, casting the mixed fuel material and metalnon-fuel material to create a fuel core stock, surrounding the fuel corestock with a cladding material, and co-extruding the fuel core stock andcladding material to create the fuel element; and mounting the pluralityof elongated fuel elements to a frame of the fuel assembly.
 2. Themethod of claim 1, wherein: the plurality of elongated fuel elementscomprise a plurality of elongated metal fuel elements; and the fuelmaterial comprises metal fuel material; and the fuel core stockcomprises a metal fuel core stock comprising an alloy of the metal fuelmaterial and the metal non-fuel material.
 3. The method of claim 1,wherein the fuel material comprises ceramic fuel material.
 4. The methodof claim 1 further comprising providing additional fuel elements in thefuel assembly, the additional fuel elements having a configurationdifferent from a configuration of the plurality of elongated fuelelements, and wherein the plurality of elongated fuel elements providedat least 60% of a total volume of all fuel elements of the fuelassembly.
 5. The method of claim 1, wherein an average thickness of thecladding after co-extrusion is at least 0.6 mm.
 6. The method of claim1, wherein casting the mixed fuel material and metal non-fuel materialto create the fuel core stock comprises creating adisplacer-blank-shaped void in the fuel core stock, the method furthercomprising: positioning a displacer within the void in the fuel corestock after casting the fuel core stock.
 7. The method of claim 1,wherein: the frame comprises a lower nozzle that is shaped andconfigured to mount to a core of a land-based nuclear power reactor; andthe fuel assembly is thermodynamically designed and physically shapedfor operation in the land-based nuclear power reactor.
 8. The method ofclaim 7, further comprising placing the fuel assembly into theland-based nuclear power reactor.
 9. The method of claim 1, wherein: amoderator:fuel ratio is an area ratio within a cross-section that isperpendicular to longitudinal axes of the plurality of fuel elements andextends through the plurality of fuel elements, the ratio being a ratioof (1) a total area available for moderator flow for the plurality offuel elements to (2) a total area of the fuel kernels of the pluralityof fuel elements; and the moderator:fuel ratio is 2.4 or less.
 10. Themethod of claim 1, wherein the step of manufacturing elongated fuelelement includes forming each of the elongated fuel elements with aspirally twisted, multi-lobed profile that defines a plurality of spiralribs.
 11. The method of claim 10, wherein the multi-lobbed profilecomprises lobe tips and intersections between adjacent lobes, andwherein the cladding material is thicker at the tips than at theintersections.
 12. The method of claim 10, wherein, during the step ofmounting, the ribs of adjacent fuel elements periodically contact eachother over the axial length of the fuel elements, such contact helpingto maintain the spacing of the fuel elements relative to each other. 13.The method of claim 1, wherein the step of mixing further includesmixing burnable poison with the fuel material and the metal non-fuelmaterial, and wherein the step of casting further includes casting theburnable poison with the mixed fuel material and metal non-fuel materialto create the fuel core stock.
 14. The method of claim 1, wherein themetal non-fuel material comprises at least one of zirconium, aluminum,and a refractory metal.
 15. The method of claim 1, wherein the framecomprises a lower nozzle that is shaped and configured to mount to acore of a land-based nuclear power reactor, and wherein the fuelassembly is designed and physically shaped for operation in theland-based nuclear power reactor.
 16. The method of claim 1, wherein thecladding material is metallurgically bonded to the fuel core stockduring the step of co-extruding.
 17. The method of claim 1, wherein thefuel material comprises plutonium, wherein the metal non-fuel materialcomprises zirconium, and wherein the metal non-fuel material comprisesbetween a 70% and a 97% volume fraction of the mixed fuel material. 18.The method of claim 1, wherein the fuel material comprises a combinationof uranium and thorium; plutonium and thorium; or uranium, plutonium,and thorium.
 19. The method of claim 1, wherein the fuel materialcomprises δ-phase UZr₂.
 20. The method of claim 1, wherein the fuelmaterial is enriched to 20% or less by uranium-235 and/or uranium-233.